Yes, hydro, geothermal are rotating machinery: so are wind turbines (although they may not be straightforward induction generators). Some countries are now occasionally getting up near 100% wind from time to time without grid collapse: agree that the countries that regularly get to 100% RE usually are mostly hydro/geothermal. What hasn't been done AFAIK is run a (large) grid entirely on PV inverters or HVDC inverters.

Voltage lagging/leading current is a question of what the load is: whether it is capacitive or inductive. You can power either with an inverter. Not sure why there would be any more losses over and beyond the ones that are already involved in PV generation with inverters.

Posted By: SteamyTeathermal losses once you start to add in thousands, or millions even, of PSUs.

Losses matters less if the marginal cost of energy (cost of units of energy captured beyond the capex fixed finance cost of PVs, turbines etc) is nearly free. Different from fuelled generation, where the marginal cost (£ and CO2) of generation is considerable per unit generated.

Posted By: finnianhydro ... are rotating machinery: so are wind turbines

but at uncontrollable frequency, so that doesn't help.

But speed must be related to flow rate? Therefore an exact power output, no more no less, regardless of demand and/or available head?

It's a request for explanation - which happens extremely well on GBF

So water in exactly matches instantaneous load, to maintain revs constant?

Obviously inverters are capable of running without an external AC grid. This is how off-grid houses and microgrids work. They are also capable of producing and absorbing reactive power, which is why you can run a washing machine in an off-grid house.

Certainly rotating machinery is the traditional way to run grids, and stability is conceptually explained to students in terms of these rotating machines, but that doesn't mean it is the only way to do it! An inverter is quite capable of reacting the same way as a synchronous motor or generator to load variations.

Posted By: barneyProvided it has a stable grid to synchronise with

I haven't heard it explained yet why it has to be a 'stable grid' i.e. totally dominated by actual rotating generators - rather than a rock-stable, same-everywhere 'heartbeat' electronic signal - which obviously can't be 'bent' while modifying any new asynchronous input.

There's a thesis at http://ethesis.nitrkl.ac.in/4685/1/211EE2330.pdf that outlines some of the techniques (complete with some formulae too)

I thought you didn't want a rotational grid, Tom - just millions of Invertors - saving the planet one roof at a time

Barney

Posted By: ringiSomething that is being missed, it cost little (or nothing) more for a rotating steam or hydro power station to be built so it can help regulate the frequency of this grid. However an inverter will need lots of quick response storage (super caps for example) along with a lot more expensive power electronics and safety systems. So far wind farms have not been willing to install such invertors.

I'd think there's a fair amount of rotational energy stored in wind turbine blades. I don't know how the inverters in a typical large wind turbine are set up but it wouldn't surprise me if they didn't already contribute to grid stability or could be made to do so with a small matter of programming.

There's a lot to be said for electronically synchronizing the grid. The other year there was a 36 hour power cut here in Caithness because lines up from Perth to Inverness went down (combination of wind and snow, I think). Shame to be sitting in the dark with a county full of wind turbines stopped when electronic synchronization could have kept a local grid running and in sync ready to reconnect.

Just to be clear, the tight limits on frequency and voltage for distributed generation are set by regulation by national grid. Distributed generation is designed to go offline first.

They have nothing to to with the capabilities of inverters and also apply to micro-hydro and household-scale wind as well as PV.

Distributed generation is forbidden to output to the grid if the grid goes into a fault condition. This is a matter of regulation rather than capability.

The biggest stresses on the grid arise due to trips of large single generators (i.e. coal or nuclear), or a link going down. Suddenly you need to find an extra GW.

Country-scale grids have operated almost entirely on a mix of PV and wind for hours at a time without the world ending, so I think that the difficulty of this is being somewhat exaggerated.

The problem of storage/intermittency is really much harder than stability, and having large scale storage would immediately allow you to solve the stability problem.

Large scale storage is all-but taken care of already
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14310&page=5#Item_6 - for example
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14413&page=3#Item_5

Not Tesla again, Tom - surely you aren't reasonably suggesting that we use electrochemical storage ?

I guess then Ed, you've just made the case for having a big stable grid

Basically, what we have is a grid system designed for large scale generation, transmission, distribution and consumption - it handles voltage control and frequency stability very well, it is responsive to short term load fluctuations, medium term load shifts, fault clearing capability and a mix of generation types

Now we want a grid that massively distributes generation, removes a significant amount of inertia in that generation, expects systems designed for distribution to now act as transmission and expects transmission systems to be able to move power through when generation and load are significantly displaced

It's why the massive wind generation in north Germany is a major problem for the grid operators that requires rerouting through Poland and Austria to south Germany where the load is - that's the underlying reality

I'd be perfectly happy with a redesigned, fit for purpose new grid system (shall we say circa Euro 500 billion) - what we can't do is keep increasing the penetration of wind and solar without maintaining enough high inertia plants to maintain stability and providing in country bypass (HVDC) to allow transfer
Regards

Barney

Posted By: barneyI guess then Ed, you've just made the case for having a big stable grid

Nobody's disputing that that's a good thing. The question is whether it necessarily requires large amounts of rotating machinery to obtain that stability.

I was thinking more of pan Europe with the 500 billion, Ed, UK only would be cheaper (but look at that in the context of the outrage at HPC costs) - the idea that somewhere the sun is always shining or the wind blowing and we would need to export or import over much longer distances - seems to be close to figures from European (including UK) wind and solar studies.

I did add a rough order figure however for storage systems which we are likely to need

In terms of rotational plant - I guess we will always have a differing viewpoint - from all sorts of studies I've seen over a number of years, stability tends to diminish as you add more invertor based technology and less rotational units - basically the torque aspect required for ride through, the variability of the power source (input power) and the inability to operate other than inside quite tight tolerances with each other or with a grid

If we accept that invertor based systems are deployed on the ends of distribution systems and are not in the control of grid operators, then they will always need a stable power source to allow synchronization.

If we group them together in "farms" or "plants" (a bit like traditional generation) that's a different issue - but that's not where we are going at the moment with Tom's "autonomous grid" (I've read that as we all have a bit of generation and we make that available to a wider grid)

Given the variable and uncontrolled input I don't see how we can avoid "traditional" rotating plant (but we can spin that plant from greener fuels which could be wind or solar via gasification or into motor generators, water storage etc)

It might be worth repeating the question - can the UK meet it's energy needs from autonomous, wind and solar with an entirely invertor based grid even if that needs HV DC interconnection from Europe (regardless of the generation source) for when it's dark and still

Would we need to impose rotational plant for stability (driven from storage) ?

Regards

Barney